Process for dehydration of acetic acid and other lower fatty acids



Patented May 9, 1939 v UNITED STATES PROCESS FOR DEHYDRATION OF ACETIC ACID AND OTHERLOWER FATTY ACIDS Donald F. Othmer, Brooklyn, N. Y., assignor to Tennessee Eastman Corporation, Kingspprt, Tenn., a corporation of Virginia No Drawing. Application November 30, 1934, Serial No. 755,361

15 Claims.

This invention relates to a process for the concentration of aqueous solutions of the lower fatty acids, for example, acetic acid, the removal of part or all of the water therefrom, and the production thereby of a. concentrated acid.

Of the several processes for concentration of acetic acid from water by the selective and counter current extractive action of an organic solvent, the following may be mentioned:

The use of a low boiling solvent such as ether or chloroform which may readily be separated from the concentrated acid after the extraction operation by fractional distillation.

The use of intermediate boiling solvents such as butyl acetate which may be used as azeotropic withdrawing agents and separated from the concentrated acid according to the method of my co-p'ending application Serial Number 734,991, now Patent No. 2,050,234.

The use of intermediate boiling solvents in which the water in the mixture of solvent, acid, and water obtained by the extraction is removed by azeotropic distillation with a suitable added material, and the acid by azeotropic distillation with a third added material. The use of solvents of sufficiently higher boiling point than acetic acid, so that after extraction of the dilute acid in either the liquid or the vapor phase, the acetic acid and water may be removed from the solvent by fractional distillation.

It is to the latter class of extraction methods that my invention belongs; and I have found that by taking advantage of the several physical properties of certain high boiling organic liquids, acetic acid or other lower fatty acid or a mixture of two or more of the lower fatty acids may be conveniently and cheaply separated from part or all of the water associated therewith, in a novel process combining several methods, well known and understood by those skilled in the handling and processing of liquid products.

Among the material which I have found particularly suitable for this concentration by extraction may be mentioned the high boiling alcohols such as caprylic and other octyl alcohols and others of from six to ten carbon atoms both aliphatic and aromatic; the esters of these alcohols with the lower fatty acids themselves; the mixed ethers and ketones of these higher alcohols with the lower four members of the alcohol series; the esters of the four lower alcohols with certain dibasic acids such as oxalic, sebacic, and phthalic; and certain esters such as those of the lower alcohols and phenyl acetic acid, and of the lower fatty acids with phenol, methyl phenyl carbinol (a compound having the formula,

or tolyl carbinol. When I employ the term, high boiling point solvent, I refer to agents such as described in the preceding paragraph.

In the concentration of the dilute fatty acid, for example acetic, the aqueous solution is systematically extracted in a suitable extracting apparatus so that the water layer discharging is substantially free of acid, and the solvent layer contains all of the acid entering in the dilute feed.

Because of the very low mutual solubilities of my preferred materials with water, the amount of water dissolved with the acid in the solvent layer is in every case very low. The ratio of acid to water is thus very high; and expressed on the basis of acid strength (neglecting solvent present) the acid can usually be obtained at a strength of from '75 to 95% at the end of this single extraction operation. Earlier processes have required two distinct extraction operations with different solvents to obtain acid of such high concentration before distillation.

The removal of the acetic acid and the small amount of water from the solvent layer after extraction is best accomplished by distillation, either with or without rectification. In the use of the higher boiling members of my preferred group of solvents, I have found that the water and acetic may be removed together by simple distillation with little or no rectification because of the great difference in the respective boiling points. The simplification of plant equipment and operation so obtained whenthe boiling point of the solvent is above/200 or 250 degrees centigrade is considerable over earlier processes requiring expensive and involved rectifying operadilute solution consisted of a pure acid and water,

, the concentrated acid obtained by simply distilling it off the solvent is satisfactory for use without further processing.

When using some of the lower boiling members of my group of preferred solvents, I have found it necessary after removing the bulk of the acid to employ one of several methods to satisfactorily remove the last traces of acid and in order to obtain acid free solvent for reuse. This may be done simply: (a) by rectification with a rectifying column, of a length and efficiency much less than that required by the usual acid concentrating processes (b by vacuum distillation, either simple or with N e use of a short rectifying column (c) by treating the solvent and residual acid ,with saturated or superheated steam.

In this latter method, the dilute acid produced by this steam distillation, after the most of the acid has been removed in a concentrated form, is kept separate after condensation and returned to the extraction equipment along with the dilute feed. The expense of this steam is in every case small; and if the process is operated continuously, after the main distillation of acid in a first still, this steaming operation is conducted in a second still with or without a rectifying column attached.

In those cases where a stronger acid is required than that produced directly by the extraction, it may usually be made readily by ordinary rectification methods, since the amount of heat required to produce even anhydrous or glacial acid from a strength of '75 to 95% is not excessive. I have discovered that this rectification operation may be conducted in part by the heat in the vapors formed on the distillation of acid and residual water from the solvent. These vapors may be passed into a mid-point of a distilling column operated in the usual manner and the water removed from the top as a dilute acetic acid which may be returned, if desired, to the extractor, while a more concentrated acid is drawn off of the base. In the case where acid free water is desired at the top of this distilling column, a suitable azeotropic withdrawing agent may be employed, selected from the group in my co-pending applications 703,317, now Patent No. 2,076,184; 703,556, and 734,991, new Patent No. 2,050,234.

I have found that the higher alcohols such as caprylic or other of the octyl alcohols may be used directly in the process either singly or mixed with their isomers or other alcohols of different molecular weight. The high degree of insolubility in water and the excellent distribution coefllcient of acetic acid between these alcohols and water makes them very suitable. After a time in the system, the alcohol or alcohols may become esterlfied with the particular acid or acids being handled; and since the respective esters may be used equally well, the process is conducted with any mixture of ester and alcohol which may eventuate after a final steady operating state has been reached.

My preferred materials may be used in a vapor state extraction operation in which the liquid extractant descends in an extractor comprising one of the several standard designs of rectifying column, against a rising stream of vapors ofdilute acid. The dilute acid, if originally obtained as a liquid, is vaporized to pass to the extractor column, and any solid materials originally present in solution or suspension are left behind. The water vapor passes out the top of the column unabsorbed by the extracting. solvent, and the acid-solvent mixture which collects at the base is separated in the same manner as that from a liquid-liquid extractor, after suitable decantation of any water layer which may also be collected at the base of the vaporous extracting column.

I have also found that in case of some of my preferred materials, notably some of the esters of other acids than the ,acid or acids handled, there is a certain amount of decomposition of the solvent due to. the continual contact with acetic acid. Di-butyl phthalate, one of my preferred materials, has excellent extractive powers, a distribution coeilicient of over 4 to 1 in favor of the solvent layer, and an acid strength of over being obtained; but in its use, there is a certain amount of conversion of the ester to butyl acetate and phthalic acid. Under some conditions, I have found it practical to remove these products of ester interchange or other'products of solvent decomposition for use in other operations, and add a few percent of fresh solvent from day to day as it becomes necessary to make up for the solvent decomposed.

Inasmuch as I have taught that it is preferable to remove the products of solvent decomposition, the various ways in which the decomposition products may be removed in any particular in-- stance will be readily arrived at by one skilled in the art. In'the instance set forth above, the decomposition products, butyl acetate and phthalic acid, may be removed in a number of ways. For example, butyl acetate may be distilled oil! with water and acetic acid and may be further separated or otherwise treated as set forth in my United States Patent 2,050,234. The butyl acetate being so much more volatile than solvent would offer no difliculty in separation. The phthalic acid, on the other hand, would remain since it is substantially non-volatile under the aforementioned conditions. This acid may be removed, for example, by the addition of basic material after acetic acid has been distilled. This would give a salt of phthalic acid which under the conditions would be insoluble and could be removed by any conventional procedure such as precipitation or filtration.

I have also found that circulating hot oils, hot water at high pressures, and heating with the vapors of mercury or high boiling organic materials of the nature of di-phenyl or di-phenyl oxide are suitable methods of surface heat transfer to obtain the high temperatures necessary to accomplish the removal of acid from, and/or distillation of, my preferred solvents.

It will be understood by those skilled in the art that many arrangements of standard equipment may be used to carry out the features of my invention, and are within the spirit of my disclosure as limited and defined by the appended claims.

In particular, it may be noted that any standard type of extractor and distilling columns or other equipment, efficient ,for this purpose may be used,.that dilute solutions of any of the four lower fatty acids or any' combination of two, three or four may be concentrated; that anycombination of liquid and/or vaporous methods of introducing the dilute acid to be concentrated commonly employed by those skilled in the art may be used in the extractor or distilling columns; that any small amount of extracting solvent brought over in a steam distillation with the water may be recovered and returned by known methods, as well as that necessarily present in very dilute solution in the waste liquid discharged from the extractor; that various mixtures of my preferred materials may be used in combination as well as singly; that the acid produced may be either partly or completely dehydrated and passed from the distilling system in either a liquid or a vaporous condition; that any temperature up to the boiling point of the dilute acid may be used for liquid-liquid extracting totake advantage 0 that temperature at which extraction or disbution coefficient is most favorable, or where lvent body is liquid or has favorable viscosity; t t any combination of heat recovery equipment or methods known to the art may be employed in conjunction with preheating liquid feeds, etc., in utilization of the heat present at the high temperatures necessarily employed; and that any degree of pressure or vacuum which is practical on an industrial scale may be maintained in either the extraction or distillation parts of the operation.

Having described my invention, what I claim and desire to secure by-Letters Patent is, as

follows: I

1. The process for obtaining substantially anhydrous acid from aqueous solutions containing at least one of the lower fatty acids by'an extraction treatment with a high boiling point solvcnt of sufiiciently higher boiling point than the acid so that after extraction the acid and any water may be removed from the solvent by distillation which comprises extracting the aqueous lower fatty acid solution with the high boiling point solvent, recovering the'extract, subjecting this extract to a distillation treatment to recover a partially concentrated acid of '75 per cent to 95 per cent strength therefrom, subjecting this partially concentrated acid to a rectification treatment to obtain substantially anhydrous acid therefrom, this rectification treatment being conducted, at least in part, by heat supplied in a preceding step of the process.

2. The process for obtaining substantially anhydrous acetic acid from an aqueous solution containing acetic acid by extraction with a high boiling point solvent of sufiiciently higher boiling point than the acetic acid so that after extraction the acetic acid and any water may be removed from the solvent by distillation which comprises extracting the aqueous acetic acid solution with the high boiling point solvent, recovering the extract, subjecting this extract to adistillation treatment to recover' a partially concentrated acetic acid of about '75 per cent to about 95 per cent strength therefrom, subjecting this partially concentrated acetic acid to a rectification treatment to obtain a substantially anhydrous acetic acid therefrom, this rectification treatment being conducted, at least in part, by the heat supplied in the\preceding-step of the process.

3.\A process for obtaining substantially anhydrous acid from an aqueous solution containing at least one of the lower fatty acids which comprises extracting the aqueous solution with a high boiling point solvent, separating the resulting extract, subjecting this extract to a distillation treatment to produce a partially concentrated acid therefrom. subjecting this partially concentrated acid to a rectification treatment whereby water is removed from the partially concentrated acid in the form of a head product of dilute acidreturning this dilute acid to the extraction step and recovering substantially anhydrous acid as the tail product of this rectification step.

4. A process for obtaining substantially anhydrous acetic, acid which comprises extracting the aqueous solution with a high boiling point solvent, separating the resulting extract, subjecting this extract to a distillation treatment to produce a partially concentrated acetic acid therefrom, subjecting this partially concentrated acetic acid to a rectification treatment whereby water is removed from the partially concentrated acetic acid as a head product in the form of dilute acetic acid, returning this dilute acetic acid to the extraction step and recovering substantially anhydrous acetic acid as the tail product of this rectification step.

5. A process for concentrating an aqueous solution containing at least one of the lower fatty acids which comprises extracting the aqueous solution with a highboiling point solvent, separating the extract, removing the bulk of the acid from this extract by distillation and removing at least a part of the acid remaining in the solvent by steaming.

6. A process for concentrating an aqueous solution containing acetic acid which comprises extracting the aqueous acetic acid solution with a high boiling point solvent, separating the extract, removing the bulk of the acetic acid from this extract by distillation and removing at least a part of the acetic acid remaining in the solvent by steaming.

7. A process for concentrating an aqueous solution containing at least one of the lower fatty acids which comprises extracting the aqueous solution with a high boiling point solvent, recovering the extract containing solvent and acid, subjecting this extract to a distillation treatment to remove the bulk of the acid from the solvent, subjecting the solvent to a steaming to remove at least a part of the residual acid and returning the acid resulting from this steaming to the extraction step.

8. A process for concentrating an aqueous solution containing acetic acid which comprises extracting the aqueous solution with a high boiling point solvent, recovering the extract containing solvent and acetic acid, subjecting this extract to a distillation treatment to remove the bulk of the acetic acid from the solvent, subjecting the solvent to a steaming to remove at least a part of the residual acetic acid and returning the acetic acid resulting from the steaming to the extraction step.

9. A process for obtaining substantially anhydrous acid from an aqueous solution containing at least one of the lower fatty acids including the employment of a high boiling point solvent of sufflciently higher boiling point than the acid so that after extraction the acid and any water may be removed from the solvent by distillation which comprises extracting the aqueous solution with the high boiling point solvent, separating the extract, subjecting this extract to a distillation treatment to produce a partially concentrated acid therefrom, subjecting this partially concentrated acid to a rectification treatment in the presence of a suitable azeotropic withdrawing agent whereby acid-free water is obtained as the head product and a substantially anhydrous acid as a tail product.

10. A process for dehydrating an aqueous solution containing at least one of the lower fatty acids, including the employment of a high boiling point solvent of sufllciently higher boiling point than the acid so that after extraction the acid and any water may be removed from the solvent by distillation, which comprises extracting the aqueous acid solution in the liquid phase under at least atmospheric pressure and at a temperature of almost the boiling point of the dilute acid under treatment, separating the resulting heated extract and immediately recovering the dehydrated acid therefrom by distillation.

11. The process for concentrating an aqueous solution containing at least one of the lower fatty acids which comprises extracting the aqueous solution with an agent essentially comprising octyl alcohol.

12. The process for concentrating an aqueous solution containing at least one of the lower fatty acids which comprises extracting the aqueous solution with an agent essentially comprising a -mixed ether of octyl alcohol with an alcohol from the lower four members okthe alcohol series.

13. The process for concentrating an aqueous solution containing at least one of the lower fatty acids which comprises extracting the aqueous solution with an agent essentially comprising an ester of the four lower alcohols with an-aliphatic dibasic acid.

14. The process for dehydrating aqueous solu- 19 tions containing at least one lower fatty acid by extraction with a highboiling point solvent that decomposes to some extent during continual contact with lower fatty acids, which comprises extracting aqueous solutions containing at least 

